1. Field of the Invention
The present invention relates generally to threaded couplings used for joining sections of tubing, casing, line pipe or other tubular sections forming long strings of pipe used in drilling and producing oil and gas. More particularly, the invention relates to a threaded coupling for joining tubulars used in offshore production of oil and gas, designed to reduce or prevent corrosion from free exchange of seawater into the connection.
2. Description of Related Art
In the search for oil and gas reserves, major oil companies have increasingly moved to deep water drilling environments. Early success in deep water has demonstrated the existence of large oil and gas fields. However, the deep water drilling environment has presented new industry challenges that make these endeavors both risky and expensive.
Tubular strings running from a drilling rig or production platform to the sea floor are known as risers. These are the conduits for bringing oil and gas to the surface. Each riser string consists of individual, 40-foot long tubular sections joined together with a variety of threaded couplings. Other installations use specially rolled longer tubular sections (i.e., greater than 40 feet in length) or welded, double joint assemblies having connections at both ends. On the rig, each tubular section is raised in the derrick, lowered below the deck and held at the floor with a threaded coupling facing up. The next tubular section is raised into the derrick and stabbed into the coupling. After successful stabbing, the tubular section is screwed into the coupling. All assembled tubular sections are then lowered in preparation for the next tubular section. The process continues, one tubular section at a time, until the string connects to the well at the sea floor.
Several types of riser strings are common. One type is a drilling riser that is temporary and only in service for a few weeks or months. Another is a production riser used for producing oil and gas. Production risers can be in service for many years. Other types of risers include intervention and work-over risers.
While in service, riser strings are exposed to a variety of cyclic loads and the corrosive effects of seawater. These strings are subject to tension, compression, bending, and pressure (internal and external) loads commonly referred to as cyclic loading. In deep water, drilling rigs and production platforms are floating vessels. While maintained in a relatively stationary location through various anchoring techniques, these vessels are constantly in motion. Thus, riser strings are connected to a moving vessel on top and to a stationary xe2x80x9ctemplatexe2x80x9d on the sea floor.
Movement of the vessel due to wind, tides, storms, eddy currents and other forces is three dimensional. The rig moves up and down and in a random motion about the well center. This constant movement subjects the riser strings to constant tension, compression and bending. Seawater exerts external pressure on the outer surface and the flow of oil and gas exerts internal pressure. Temperature changes occur with production that can elongate or shorten the riser string and increase these loads. While each of these loads is generally low level compared to the working strength of the materials used, the movement over time can ultimately cause failure due to material fatigue. Over prolonged service, these riser strings may experience significantly higher load levels for short periods such as during hurricanes. While available materials can be selected to resist anticipated static loads for the intended service life, long term cyclic loading and other factors may cause early failures.
Long term exposure to seawater may cause corrosion of the tubular sections and couplings used in riser strings. The free exchange of seawater into the threaded region of the coupling allows the entry of oxygen which promotes corrosion that adversely affects fatigue life. As a preventative measure, tubular sections may be protected with a variety of corrosion resistant coatings. Additionally, sacrificial anodes that are electrically connected between the tubular sections and the connections may be used to provide cathodic protection against corrosion due to prolonged seawater exposure.
Each pin end is externally threaded and the box end is internally threaded. The pin threads are machined into the tubular section and taper from its outer diameter to its end, or the pin face. Each pin end resembles a truncated cone with helical threads around its outside surface. The truncated cone has an outer diameter smaller than the tubular section at the pin face and then tapers out to the fall outer diameter. Threading begins at the pin face and runs out until the thread root diameter exceeds the tubular outer diameter. Thus, threads closest to the pin face and continuing around the pin end for most of the threaded area are known as xe2x80x9cperfectxe2x80x9d or full-crested threads. The number of these perfect threads is taper dependent.
Beyond the perfect threads, imperfect threads are formed. These are threads with crest diameters that exceed the tubular outer diameter and are no longer full-crested. Imperfect threads run out when the thread root diameter exceeds the tubular outer diameter. The appearance of imperfect threads ranges from near full-crested to barely visible scratches in the outer diameter surface of the tubular section. Pin threads are generally xe2x80x9cas machinedxe2x80x9d and are not coated with any protective material.
Couplings for joining together tubular sections have opposing internally threaded box ends, and generally are larger in diameter than the tubular sections. The thread form and taper of each box are typically the same as those of the pin ends of the tubular sections. Unlike pin threads, box threads are all perfect threads. Additionally, unlike pin threads, box threads are coated with a variety of materials which are typically corrosion resistant and are used to resist thread galling with repeated make-ups and break-outs.
When assembled, there exists a void between the perfect box threads and the mating imperfect pin threads. This void exists at the mouth of the coupling on assembled connections. The xe2x80x9cas-machinedxe2x80x9d imperfect pin threads in the void are exposed to seawater and, in time, experience corrosion pitting. Seawater migrates into the connection to cause corrosion. More specifically, free exchange of seawater into the connection after the connection has reached equilibrium at a sub-sea depth allows entry of oxygen through the imperfectly threaded region. As a result, pinpoint corrosion pitting occurs, significantly reducing the fatigue life of the connection and increasing the risk of early failure in the form of fractures.
Threaded tubular connections are assembled with a variety of lubricating compounds. In addition, couplings often have positive seals intended to prevent internal pressure leaks. Some of these seals also are intended to prevent external leaks. However, during make-up, excess thread lubricant is forced forward toward the center of the coupling and backward toward the coupling face. In the center, lubricating compound that doesn""t squeeze out into the inner diameter of the tubular section resides in the threads. After some time, this excess squeezes out of the connection in the other direction toward the coupling face as the connection settles after make-up and reaches equilibrium.
Couplings with external pressure seals, however, can block the exit channel for excess thread compound. As a result, thread compound can be trapped in the threads. If it is trapped in the threads, thread compound can generate significant hydraulic pressure tending to jack the pin away from the box. This can result in significant stresses that may diminish connection performance in service and lead to failures. Thus, there is a need to avoid trapping excess thread compound in a connection with both internal and external seals.
The present invention overcomes the above problems and disadvantages by providing a threaded coupling with a water exclusion seal system to prevent the free exchange of seawater into the connection through the critical area between the perfect box threads and mating imperfect pin threads. The water exclusion seal system includes three interrelated components: a thread compound relief groove in the coupling inner diameter, a pressure activated resilient seal, and a groove in the coupling inner diameter for the pressure activated resilient seal.
The thread compound relief groove is located in the unthreaded area between the threaded area and each end of the coupling, to provide a reservoir for excess thread compound that is squeezed from the threads during make-up. The thread compound relief groove is dimensioned to provide sufficient volume to store excess thread compound and eliminate any associated hydraulic pressure build-up in the joint during or after make-up, or any negative impact of the thread compound on the seal.
The pressure energized resilient seal is energized when it is squeezed between the tubular outer diameter and a groove in the coupling where the seal is positioned. The seal achieves contact pressure with the tubular outer diameter and the groove to form a seal that prevents free exchange of sea water in the imperfect threaded area of the tapered threaded connection.
The groove for the pressure activated resilient seal is machined into the inner diameter of the coupling between the thread compound relief groove and the end of the coupling. This groove provides an area having a radial distance from the outer diameter of the tubular section that is less than the radial thickness of the seal. The pressure activated resilient seal in this groove is compressed radially between the outer surface of the tubular section and the coupling groove, to provide effective and repeatable sealing if the connection is disassembled and later reassembled.